fix: add option to disable focused page emulation

[samiralibabic]

Summary

• Adds --emulate-focused-pages / --no-emulate-focused-pages so users can opt out of passive focused-page emulation during page snapshot setup.
• Forwards the background option when creating pages in isolated contexts.
• Documents the new CLI option in README.md.

Scope

This intentionally does not suppress explicit activation requests such as select_page with bringToFront: true.

The option only controls passive focused-page emulation from createPagesSnapshot().

Focused-page emulation remains enabled by default to preserve existing behavior for multi-agent workflows where pages should behave as focused/active. Disable it only when visible-browser focus behavior is problematic.

Validation

• npm run build
• npm run test:no-build -- tests/cli.test.ts tests/McpContext.test.ts
• npm run test:no-build -- --test-name-pattern=isolatedContext tests/tools/pages.test.ts
• npm run check-format

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[zyzyzyryxy]

Thank you for your contribution!
We'd like to understand your case better before deciding on this PR. Please file a feature request issue and describe in detail in what scenario emulating a focused page is problematic. We'll follow up with questions there if needed.